The present invention relates to a power supply circuit, and more particularly to a voltage selector circuit in the case of operating drive (or measured) means with a plurality of power sources.
In a magnetic recording/reproducing apparatus etc., a plurality of power sources of, e.g., +12 V and +5 V have heretofore been used in a circuit for moving the magnetic head of a magnetic disk device and for pressing the magnetic head against a magnetic disk, etc.
To this end, two voltage sources have been employed. When the respective voltages rise due to the turning ON or OFF of a power supply, writing or reading errors might occur depending upon whether the circuit for pressing the magnetic head operates earlier or later.
In the case of driving or measuring a field-effect transistor (FET), such as a GaAs FET or the like, with a plurality of power sources, a first voltage source connected to the drain of the FET and a second voltage source connected to the gate thereof must be applied or cut off in a predetermined sequence. More specifically, in turning off the power supply, the second voltage source is turned "on" to apply a bias voltage to the gate electrode of the FET, and the first voltage source is subsequently turned "on" to apply a bias to voltage the drain electrode thereof. In turning on the power supply, the bias voltage applied to the drain electrode is rendered "off" and thereafter, the bias voltage applied to the gate electrode is rendered "off." Otherwise, the FET will breakdown. In order to prevent breakdown of the FET, switching operations at the turning off or turning on of the power supply have been scrupulously performed. Alternatively, a sequencer circuit has been assembled of a relay, etc., so that the respective voltages may rise or fall in a predetermined sequence when the power supply is turned off or turned on. The addition of the relay, etc., to the power supply circuit of this type, however, incurs the disadvantages of being a large-sized power supply device, having low circuit reliability and having a high cost.